[Bug tree-optimization/69445] New: Fail to devirtualize call to base class function even though derived class type is 'final'

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69445

            Bug ID: 69445
           Summary: Fail to devirtualize call to base class function even
                    though derived class type is 'final'
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

G++ should devirtualize both calls in func(const C&):

struct Base {
  virtual void foo() const = 0;
  virtual void bar() const {}
};

struct C final : Base {
  void foo() const { }
};

void func(const C & c) {
  c.bar();
  c.foo();
}

We take advantage of the fact that C is final and so C::foo() can't be
overriden, but don't apply the same logic to Base::bar().

With -O3 Clang optimises func to an empty function but with G++ the x86_64 code
is:

Base::bar() const:
        rep ret
func(C const&):
        movq    (%rdi), %rax
        movq    8(%rax), %rax
        cmpq    Base::bar() const, %rax
        jne     .L5
        rep ret
.L5:
        jmp     *%rax